Separation of trioxane from liquid mixtures

ABSTRACT

Process for the removal of trioxane from a liquid mixture comprising trioxane, formaldehyde, alcohol, hemiformals formed from the form-aldehyde and the alcohol, usual secondary components arising in the preparation of trioxane, and, in addition or as an alternative to the alcohol and the hemiformals, water, in which the trioxane is transferred into the gas phase by vaporization or evaporation and subsequently converted into a liquid state by condensation and obtained as condensate or converted into a solid state by desublimation and obtained as desublimate. Through the process, an increase in the concentration of the trioxane by 2.5 times or more is achieved.

The invention relates to the removal of trioxane from a liquid mixturecomprising trioxane, formaldehyde, alcohol, hemiformals formed fromformaldehyde and alcohol, and small amounts of secondary components. Inaddition or as an alternative to the alcohol and the hemiformals, themixture may also comprise water and reaction products formed fromformaldehyde and water.

For the preparation of the engineering plastic polyacetal, in particularof polyoxymethylene (POM), high-purity trioxane is required. The qualityof the plastic, i.e. the achievable degree of polymerization, is,besides the polymerization conditions, determined principally by thepurity of the trioxane.

Various processes for the preparation of trioxane are known (for examplehomogeneously or heterogeneously catalyzed from aqueous formaldehydesolutions (AT 252913) or heterogeneously catalyzed from gaseousformaldehyde on heteropolyacids (EP 0606056). Irrespective of thepreparation process, trioxane is generally not produced as a puresubstance, but instead always as a mixture with unreacted formaldehydeand other substances, such as alcohol, water and small amounts of othercomponents, so-called secondary components, such as methanol, methylformate, methylal, formic acid, dioxolane and tetraoxane. For use of thetrioxane in the polymerization, this must be separated, in particularfrom the formaldehyde, and may only contain small amounts of secondarycomponents.

A multiplicity of literature is known relating to the removal oftrioxane from aqueous formaldehyde-containing mixtures. The separationof gaseous mixtures of formaldehyde and trioxane is only described in afew places.

The separation from aqueous solution has hitherto been carried out, inparticular, by distillation (AT 252913 and JP 83/171278). In thedistillation, unreacted formaldehyde is frequently fed back into thereactor, where it is reacted further to give trioxane. A limit for theincrease in the concentration of trioxane by distillative removal arisesthrough an azeotrope of trioxane with water which boils at 92° C. and 1bar and which generally has a trioxane content of about 70% by weight. Afurther disadvantage of this process is possible solids formation bypolymerization of formaldehyde or formation of paraformaldehyde,especially in the region of the column head. In order to avoid same, allapparatus parts must either be heated to temperatures above 100° C. (forexample at a formaldehyde partial pressure of 1 bar) or wetted with aliquid.

A further process for the separation of formaldehyde and trioxane fromaqueous solutions comprises the extraction of the trioxane with organicsolvents in which trioxane has higher physical solubility thanformaldehyde. This process has exclusively been employed for the removalof trioxane from the aqueous phase. Examples of organic extractants usedare saturated aliphatic or aromatic hydrocarbons or halogenatedhydrocarbons (EP 0583907) which are sparingly miscible or evenimmiscible with water. A disadvantage of the extraction is that anadditional substance in the form of the organic solvent is introducedinto the process, which makes subsequent work-up of the organic phasenecessary too. A further disadvantage is that large parts of thetrioxane sometimes also remain in the aqueous phase. Large amounts oftrioxane therefore have to be circulated or are lost in the work-upprocess.

A further possibility described for the selective removal from anaqueous formaldehyde/trioxane phase is crystallization of the trioxane(DE 3508668). The trioxane concentration in the aqueous mixture must begreater than 50% by weight here.

In the preparation of trioxane by trimerization of formaldehyde fromaqueous formalin solutions, the above-mentioned processes of azeotropicdistillation, extraction and, if desired, crystallization are generallylinked with one another in a suitable manner in order to obtain trioxanein the requisite high purity.

For the separation of gaseous mixtures of formaldehyde and trioxane,selective absorption of one species has frequently been employed. Ingeneral, either the formaldehyde is chemisorbed and the trioxane left inthe gas phase (GB 1245990) or conversely selective physisorption of thetrioxane is carried out (EP 0680959). Since no liquid phase in whicheither only the formaldehyde or only the trioxane is soluble has beenfound, fractions of the respective other species are also bound here.For this reason, the purities necessary for polymerization cannot beachieved using this process. Furthermore, large losses of the valuableproduct trioxane occur.

The separation of a mixture of formaldehyde and trioxane in gaseous formwith a low water content by absorption of the trioxane in an alcoholfollowed by crystallization from the alcoholic solution is described inthe as yet unpublished German Patent Application No. 19833620.9. Thisprocess forms a possible step in a novel process for the preparation oftrioxane from methanol, consisting of the steps of nonoxidativedehydrogenation (DE 3920811), removal of formaldehyde (as yetunpublished German Patent Application Nos. 19747647.3 and 19748380.1),formaldehyde trimerization (EP 0606056 and EP 0691388) and removal oftrioxane (for example in accordance with German Patent Application No.19833620.9).

The object was to find an alternative process for the removal oftrioxane from a liquid mixture comprising trioxane, formaldehyde,alcohol, hemiformals formed from formaldehyde and the alcohol, secondarycomponents and, in addition or as an alternative to the alcohol and thehemiformals, water and reaction products formed from formaldehyde andwater, in which the aim was to obtain the trioxane in high purity. Theother valuable products present in the mixture, namely formaldehyde andpossibly the alcohol, should if possible be present in other productstreams that can be utilized.

This object is achieved in accordance with the invention by transferringtrioxane as selectively as possible from the liquid mixture into thevapor phase in a suitable manner, and subsequently removing itselectively in liquid or solid form by cooling and subsequentcondensation or desublimation.

The invention therefore relates to a process for the recovery oftrioxane in which the trioxane is transferred from a liquid mixturecomprising trioxane, formaldehyde, alcohol, hemiformals formed from theformaldehyde and the alcohol, usual small amounts, formed in thepreparation of trioxane, of lower- and higher-boiling secondarycomponents and, in addition or as an alternative to the alcohol and thehemiformals, water, into the gas phase by volatilization or evaporationand subsequently converted into a liquid state by condensation andisolated as condensate or converted into a solid state by desublimationand isolated as desublimate.

The invention also relates to the use of the trioxane condensate ordesublimate obtained by the process according to the invention, which ischaracterized by its composition and a trioxane content of at least 80%by weight, for the preparation of polymers and fuels or for obtainingformaldehyde by depolymerization.

Examples of usual secondary components are methanol, methyl formate,tetraoxane, dioxolane, trioxy ether and traces of formic acid. Thealcohol present in the liquid starting mixture, which in some cases isable to form hemiformals with the formaldehyde, is preferably amonohydric alcohol, for example cyclohexanol, methanol, propanol orbutanol. However, it is also possible to use other alcohols, includingpolyhydric ones, such as glycerol, diethylene glycol, triethyleneglycol, triethanolamine, butanetriol and pentanetriol. If desired, it isalso possible to use a mixture of alcohols. The alcohol shouldpreferably have a higher boiling point and a lower melting point thantrioxane.

In contrast to German Patent Application No. 19833620.9 and DE 3508668,the process according to the invention achieves, in particular, theobject of separating the mixture in liquid form with a low watercontent, as arises, for example, in the absorption step after theprocess according to German Patent Application No. 19833620.9 and thusin the above-mentioned novel process for the preparation of trioxanefrom methanol. However, the process can also be used for mixtures havinga higher water content.

Surprisingly, it has been found that the process according to theinvention enables a high concentration of trioxane to be achieved evenfor a very low initial concentration of the trioxane in the liquidmixture.

In order to isolate the trioxane by the process according to theinvention, the trioxane is firstly transferred from the liquid mixtureinto the vapor phase at a temperature in the range from 20 to 200° C.,preferably in the range from 50 to 100° C., in a suitable apparatus withor without carrier gas. The trioxane is subsequently converted from thevapor phase into a liquid or solid physical state, i.e. condensed ordesublimed, at a temperature in the range from −20 to 113° C.,preferably in the range from 20 to 40° C. The process can be carried outat reduced pressure, atmospheric pressure or superatmospheric pressure,application of atmospheric pressure being preferred.

In contrast to DE 3508668, the removal in the process according to theinvention is preferably carried out from a low-water liquid mixture.Low-water here means that the mixture either contains no water at all,i.e. 0% by weight, or alternatively contains a maximum of 5% by weight,preferably a maximum of 3% by weight of water. This is a particularadvantage compared with the prior art, since the low water content meansthat significantly less heating energy is necessary in the process andthere is virtually no need to work up aqueous mixtures. The restrictionpresent in the process according to DE 3508668 to mixtures having atrioxane content of greater than 50% is also absent in the processaccording to the invention. In addition, however, the process accordingto the invention can also be employed for the purification of trioxanefrom aqueous mixtures which have a significantly greater water contentthan 5%.

In contrast to the process for the removal of trioxane bycrystallization in a liquid solution, either on cooled walls or insuspension, which is described in German Patent Application No.19833620.9, the trioxane in the process according to the invention isfirstly selectively vaporized or evaporated and subsequently convertedinto a liquid state (condensate) or—preferably—into a solid state(desublimate) by condensation or desublimation. Higher purities andyields can thereby be achieved than in the process according to GermanPatent Application No. 19833620.9.

Furthermore, the process according to the invention is more advantageousfrom the energetic point of view and thus with respect to the operatingcosts, since in the crystallization in accordance with German PatentApplication No. 19833620.9, cold is used, which is not necessary in theprocess according to the invention. The apparatus costs to be expectedare also lower in the process according to the invention than in thelayer crystallization described in German Patent Application No.19833620.9.

For the transfer of a trioxane from the liquid mixture into the vaporphase, it is advantageous in the process according to the invention, butnot absolutely necessary, for trioxane (boiling point at atmosphericpressure 113° C.) to have a comparatively high vapor pressure comparedwith the other components. If the cooling is carried out as adesublimation, the comparatively high melting point (about 62° C.) oftrioxane proves to be particularly advantageous. The selective removalof trioxane from the vapor phase that this makes possible means that thevapor phase can also contain lower- and higher-boiling components.

A number of possibilities exists for transferring the trioxane from theliquid phase into the vapor phase in the process according to theinvention. Thus, for example, if the trioxane content in the mixture issufficiently high, simple evaporation at atmospheric pressure can becarried out by increasing the temperature. A disadvantage here is therelatively high thermal load, which can result in liberation offormaldehyde from the hemiformal.

The transfer of trioxane into the vapor phase can also take place byatomization or evaporation under reduced pressure. Advantageous here arethe low temperatures, but possible leak problems and the requisitecomplexity for the provision of the vacuum are disadvantageous.

The use of a carrier gas, which may also be preloaded with trioxane,proves particularly advantageous in the process according to theinvention. This carrier gas is used to strip trioxane out of the liquidmixture. Both the advantageously lower temperatures and operation underatmospheric pressure are achieved in this case. In order to avoid anexcessive demand for gas, however, use should be made of a compressor,by means of which the carrier gas is circulated between the vaporizationor evaporation step and the condensation or desublimation step. Examplesof suitable carrier gases are inert gases, such as nitrogen and argon.

If the atomization or evaporation step in the process according to theinvention is carried out in a thermally gentle manner under moderatetemperatures, it proves particularly advantageous that virtually norelease of formaldehyde from the hemiformal occurs. This avoids problemswith coating formation, for example due to precipitation ofparaformaldehyde from the gas phase.

A number of apparatuses can be employed for the transfer of trioxaneinto the vapor phase in the process according to the invention. Thus,this process step can be carried out, for example, in evaporators, suchas falling-film or thin-film evaporators. However, externally heatedcolumns are also conceivable, which can be designed as spray towers andoptionally have internals, such as trays or structured packing. It isalso possible to use a heated stirred vessel or alternatively abubble-tray column, in particular if a carrier gas is used. Alsoconceivable is the use of flash apparatuses, in which superheatedsolutions are decompressed to a lower pressure.

Preference is given to apparatuses which, as standard apparatuses with asimple design and continuous operation, have a sufficiently largevapor/liquid phase interface and high throughputs. These requirementsare satisfied, for example, by evaporators, such as falling-film orthin-film evaporators.

The vaporization or evaporation can be carried out in the processaccording to the invention at temperatures in the range from 20 to 200°C., also at lower temperatures, if desired, in the reduced-pressurerange and also at higher temperatures, if desired, in thesuperatmospheric pressure range, but preferably at a temperature of from50 to 100° C. The liquid stream flowing out of the vaporization orevaporation step, which is depleted in trioxane, but comprises thevaluable products formaldehyde, alcohol and hemiformals, can be fed backinto an earlier step of the overall process described above or usedelsewhere, for example for recovery of the valuable products presenttherein. It is furthermore conceivable to feed some or all of the streamback into the vaporization or evaporation step. In the latter case, abatchwise operating procedure is preferably present.

Through the condensation or desublimation of the trioxane from the vaporphase, the trioxane can be obtained in the process according to theinvention either in the liquid state or in the solid state. If thecondensate is to remain liquid, relatively high temperatures must be setin the condensation step in view of the melting point of trioxane, butthis causes the heat and thus mass transfer and the deposition rate tobe minimized. Condensers which can be employed here are all known typesof heat exchanger with vapor-liquid phase transition; mention may bemade here merely by way of example of tube-bundle heat exchangers.

The trioxane can also be condensed or absorbed out of the gas phasecharged therewith by contact with a cold liquid. Likewise suitable forthis purpose are, for example, falling-film condensers, bubble-traycolumns or stirred vessels operated with corresponding cold liquids orsolvents. However, a disadvantage in this set-up is the purificationwhich is subsequently necessary, i.e. the removal of the trioxane fromthe liquid.

The condensation as far as the liquid phase can be carried out in theprocess according to the invention at temperatures in the range from 30to 113° C., in the case of condensation under excess pressure if desiredalso at higher temperatures, but preferably at temperatures in the rangefrom 30 to 75° C.

However, it is advantageous to use desublimation in the processaccording to the invention with utilization of the high melting point oftrioxane. This can be carried out here both on rigid, cooled walls andin a moved zone, for example in a fluid bed or a fluidized bed. Afurther advantage of desublimation is that, by varying the processparameters, specific shaping of the trioxane desublimate can be carriedout, for example deposition in a desired particle size. Thedesublimation process can be supported in the process according to theinvention by suitable measures, such as admixing a cold gas or injectinga low-boiling solvent.

If rigid walls are employed for the desublimation, the sublimate can beremoved from them either by melting or by mechanical cleaning, such asscraping-off. Mechanical cleaning is more advantageous here if theproduct must then be processed further in liquid form, since noadditional energy, as required, for example, for melting, has to besupplied, and because the process can be operated continuously. Theapparatus employed can be, for example, a scraped-shell condenser. Owingto the relatively low heat-transfer coefficient in the vapor-solid phasetransition, large exchange surfaces are generally necessary. From thispoint of view, the use of plate or finned tube heat exchangers isparticularly advantageous, from which the solid coating can be removedby melting or by sublimation, preferably supported by a temperatureincrease and a pressure reduction. Instead of plate or finned tube heatexchangers, it is also possible to employ other types of heat exchanger,for example tube-bundle heat exchangers. Operation is generally incycles, i.e. at least two heat exchangers are used in a continuousprocess.

If the desublimation is carried out in a fluid bed or a fluidized bed,higher heat exchange coefficients can be achieved in continuousoperation. Fluid bed desublimation can be carried out with or withoutaddition of a flow agent, preferably also with addition of a cooled gas.Examples of apparatuses which can be employed are fluidized-bed dryers;filtrations or sieving of the particles can, if desired, be carried outdownstream.

The desublimation can be carried out in the process according to theinvention at temperatures in the range from −20 to 70° C., under certaincircumstances also at lower temperatures, but preferably at temperaturesin the range from 20 to 40° C., since generation of low temperatures isunnecessary for this purpose, which represents a major advantage of theprocess according to the invention. If melting is necessary, this can becarried out at temperatures in the range from 30 to 113° C., also athigher temperatures in the superatmospheric pressure range, butpreferably at temperatures in the range from 60 to 80° C.

One possibility for carrying out the process steps of vaporization orevaporation and condensation and desublimation in a single apparatusarises on use of a short-path evaporator, in which the evaporation andcondensation surfaces are arranged concentrically. The short-pathevaporator allows particularly gentle handling of the product. If thecondensation surface is employed as desublimator, cycled operation isnecessary. The short-path evaporator can also be designed as a molecularcondenser or desublimator.

If the trioxane removed from the liquid mixture by a vaporization orevaporation step and condensation or desublimation step does not yethave the desired purity, multistep operation with correspondingrecycling can be implemented in the process according to the invention.It is furthermore conceivable for the product present after one step tobe subjected to fine purification with the aid of other separationmethods.

The trioxane obtained by the process according to the inventiongenerally comprises from 2 to 4% by weight of formaldehyde, from 4 to 8%by weight of alcohol and more than 80% by weight, preferably from 85 to90% by weight, of trioxane. It is suitable for all areas of application,such as polymerization to give plastics, such as polyacetals, inparticular polyoxy-methylene, the preparation of fuels and thedepolymerization to give formaldehyde, which facilitates further use forall known formaldehyde reactions, in particular if very pureformaldehyde is necessary for this purpose.

Overall, a trioxane condensate or desublimate whose trioxane contentcorresponds to 2.5 times or more the trioxane content in the liquidstarting mixture can be obtained using the process according to theinvention. However, if the trioxane content in the liquid mixture isextremely low, for example only 5% by weight, although it is notpossible to obtain a trioxane content of 80% by weight or more in thecondensate or desublimate, it is nevertheless possible to achieve anenrichment of the trioxane by tenfold, i.e. to about 50 to 65% byweight.

The process according to the invention is distinguished over theprocesses employed hitherto by the following advantages:

high purities and yields of trioxane,

in the case of aqueous mixtures, no restriction by the water/trioxaneazeotrope, in contrast to distillation,

simple apparatus implementation using standard units possible,

low formaldehyde contents, even after one process step, and thusfavorable prerequisites for further processing,

very low formaldehyde concentrations in the gas phase and thusprevention of solid or coating formation (for example paraformaldehyde),

due to moderate temperatures, thermally gentle and energeticallyadvantageous in the vaporization or evaporation step; consequentlyprevention of the release of gaseous formaldehyde,

energetically advantageous in the condensation or desublimation step dueto the possibility of using river water, recooling water or ambient airfor cooling, avoiding comparatively expensive generation of cold,

in addition energetically advantageous due to internal energy recovery,

high operational reliability,

no need for any auxiliary materials, which have to be separated off in acomplex manner,

no loss streams: discharge from the vaporizer or evaporator step can beemployed elsewhere in the process,

no formation of a preliminary fraction from a sweating process in thecondensation or desublimation step.

A possible apparatus implementation of the process according to theinvention is explained below with reference to FIG. 1.

The liquid starting mixture 1, which, besides trioxane, also comprisesformaldehyde, an alcohol (preferably having a higher boiling point thantrioxane, such as, for example, cyclohexanol), hemiformals and secondarycomponents, and in addition or as an alternative to alcohol andhemiformal, also water, is fed to the apparatus for transfer of thetrioxane into the vapor phase, the apparatus being designed here as theevaporator 2. The liquid flows through the evaporator 2 from top tobottom and exits again at the bottom as stream 3, which is depleted intrioxane. The compressor 4 conveys a circulation stream 5 of inertcarrier gas (for example nitrogen or argon) through the evaporator incountercurrent to the liquid, the carrier-gas stream being loaded withtrioxane. The loaded carrier-gas stream is passed to one of the heatexchangers 10 and 11, which are designed here as desublimers, through acorresponding position of the stop cocks 6 and 7 and the correspondingposition of the stop cocks 8 and 9. Under the prerequisite of asufficiently large temperature difference between the evaporator 2 andthe heat exchanger 10 or 11, trioxane precipitates in solid form in thefirst heat exchanger charged with gas. If this first heat exchanger isloaded, the gas stream is passed to the other heat exchanger via thestop cocks 6 to 9. The desublimate is melted in the loaded heatexchanger by an increase in temperature and can be taken off via thestop cocks 12 or 13 as stream 14 or 15. If the second heat exchanger isloaded, the gas stream is fed back to the previously emptied heatexchanger, and the desublimate is melted and taken off in the now loadedheat exchanger.

The apparatus circuit according to FIG. 1 represents only oneconceivable variant of the process according to the invention. Asdescribed above, other physical principles and apparatuses for theremoval of the trioxane by transfer from the liquid phase into the vaporphase and back to the liquid or solid phase can also be employed in theprocess according to the invention.

The fact that only moderate temperatures are necessary in the processaccording to the invention enables heat arising in other process stepsto be utilized here in an advantageous manner. If, for example, asdescribed above, the process according to the invention is combined withprior trimerization of formaldehyde from the gas phase, the heatliberated during the trimerization can be employed for heating thevaporizer or evaporator. If the heating medium leaving the vaporizer orevaporator is at a sufficiently high temperature level, it can be fed onto the desublimers for the purpose of melting the product. In this way,the thermal energy employed in the process as a whole can be utilized inan optimum manner.

The process according to the invention is illustrated below withreference to some experimental studies. The experimental set-up wascarried out in accordance with the apparatus variants shown in FIG. 1.

In all experiments, the evaporator 2 used was a thin-film evaporatorDN25 with a double-jacket evaporator body made of glass; the desublimers10 and 11 used were double-jacket tubes DN50 made of glass.

The feed 1,400 ml/h in all experiments, consisted of a liquid mixturecomprising trioxane, cyclohexanol, formaldehyde and secondary components(principally methanol and water), in which the formaldehyde waspredominantly in bound form as cyclohexyl hemiformal (results, see Table1). For comparison, a feed comprising water instead of cyclohexanol wasalso investigated (results, see Table 2).

The carrier gas used was nitrogen. All experiments were carried out atatmospheric pressure or a slight excess pressure in the nitrogencircuit. Samples were taken from the entering and exiting liquid streamsand analyzed using a gas chromatograph. All percentages in this respectare taken to mean percent by weight.

In Tables 1 and 2, the following abbreviations are furthermore used:

FA formaldehyde TOX trioxane

MeOH methanol CHOL cyclohexanol

H2O water

TABLE 1 Experimental parameters and results in the experiments withlow-water feed Temp. Temp. Desublimate jacket jacket Carrier gas FeedCycle Flow Experi- evaporator desublimer N₂ FA TOX CHOL time rate FA TOXCHOL ment ° C. ° C. l/h % % % h g/h % % % Desu 3A 60 2 300 16.2 31.550.4 0.5 53 2.5 87.3 5.1 Desu 3B 80 25 300 16.3 30.9 50.7 0.5 103 3.185.9 7.5 Desu 3C 60 2 300 23.4 30.9 43.5 0.5 52 2.6 88.3 4.0 Desu 3D 8025 300 23.5 30.7 43.5 1 98 3.4 87.4 5.7 Desu 3E 80 2 300 24.2 29.0 44.60.3 63 3.0 87.7 4.3 Desu 4A 80 25 300 21.5 30.8 45.9 0.5 101 3.3 88.06.3 Desu 4B 80 25 450 21.5 30.8 45.9 0.5 115 3.0 87.5 7.4 Desu 4C 80 25200 21.5 30.8 45.9 0.5 84 3.4 88.4 5.6 Desu 4D 80 25 300 18.8 31.2 48.20.5 99 2.6 88.4 6.9 Desu 4E 80 25 300 18.9 30.9 48.3 1.5 103 2.8 88.07.3 Desu 5A 80 25 300 17.6 39.9 39.6 0.5 118 3.0 87.7 5.8 Desu 5B 75 25450 19.5 31.8 47.0 0.5 95 2.4 88.9 7.0 Desu 5C 75 25 450 19.5 31.8 47.01.5 96 2.7 88.1 7.5 980707A 75 25 300 15.5 28.9 54.6 1 83 2.8 85.5 8.6980707B 75 25 300 15.5 28.9 54.6 0.25 80 2.8 84.6 9.7 980707C 85 25 20015.5 28.9 54.6 0.5 100 3.5 83.1 9.6 980707D 85 25 400 15.5 28.9 54.6 0.5125 3.5 78.4 14.7 980708A 85 25 300 14.6 29.0 54.2 0.5 111 3.2 82.6 11.0980708B 85 25 350 14.6 29.0 54.2 0.5 120 3.3 81.3 12.4 980805A 70 5 30024.9 5.3 66.6 1 21.3 10.7 64.6 11.2 980805B 60 5 300 20.1 3.9 73.5 216.4 12.1 51.7 21.0 980805C 60 5 300 21.5 4.4 71.5 2.17 19.3 12.2 53.721.0 980805C 60 5 300 23.1 3.2 71.6 5 15.3 12.0 55.6 20.2 980805C 60 5300 23.6 3.3 71.1 2.33 12.9 12.7 51.7 22.3

TABLE 2 Experimental parameters and results in the experiments withwater-containing feed Temp. Temp. Desublimate jacket jacket Carrier gasFeed Cycle Flow Experi- evaporator desublimer N₂ FA H2O MeOH TOX timerate FA H2O MeOH TOX ment ° C. ° C. l/h % % % % h g/h % % % % 980714A 7020 300 10.1 13.3 4.2 72.4 0.5 154 2.3 18.2 2.6 76.9 980714B 60 20 30010.1 13.3 4.2 72.4 0.5  90 2.2 17.5 2.5 77.8

In all experiments, the trioxane was obtained as needle-shapeddesublimate; the needles formed a braid and in some cases bridges overthe cross section of the desublimers. These did not hinder the flow ofthe circulated carrier-gas stream. Instead, the braid advantageouslyprevented discharge of solid particles from the desublimer with the gasstream, which rendered superfluous the use of a downstream filter orseparator, as is entirely conventional in other desublimers.

In the examples described above regarding the process according to theinvention, surprisingly high purities and yields were obtained in spiteof the simple, one-step set-up. Thus, it was possible to concentratetrioxane from low-water solution from about 30% by weight to about 90%by weight, with yields of about 80% being achieved. The formaldehyde isgreatly depleted in the desublimate compared with the feed, which isadvantageous for further processing of the desublimate. The dischargefrom the evaporator is highly depleted in trioxane. Thus, for example,only 9.0% of TOX with 28.4% of FA and 61.0% of CHOL are found in thedischarge in Experiment DESU4B; the discharge in Experiment 980708Acontains only 6.7% of TOX with 19.7% FA and 72.2% of CHOL.

A further surprising fact in the experiments was that no preliminaryfraction, i.e. no condensate from a sweating process, had to be takenoff during melting of the desublimate, but instead the entire contentsof the desublimer were usable as product. This results in anadvantageous increase in the throughput with a reduction in the cycletimes. If, by contrast, a preliminary fraction is taken off duringmelting, the purity of the product can advantageously even be increasedin the process according to the invention.

Instead of the apparatuses used in the examples mentioned, all otherapparatuses described above are also conceivable for the experiment.Thus, for example, a falling-film evaporator can be employed instead ofthe thin-film evaporator. The desublimation can also be carried out, forexample, using thinned tube heat exchangers or using fluid-bedapparatuses.

List of Reference Symbols

1 Feed line for the liquid starting mixture

2 Evaporator

3 Take-off of the trioxane-depleted liquid after the evaporator

4 Compressor for carrier gas (circulation gas)

5 Carrier-gas stream (circulation gas)

6 Stop cock

7 Stop cock

8 Stop cock

9 Stop cock

10 Heat exchanger (desublimer)

11 Heat exchanger (desublimer)

12 Stop cock

13 Stop cock

14 Take-off of the product from the heat exchanger (desublimer)

15 Take-off of the product from the heat exchanger (desublimer)

What is claimed is:
 1. A process for the recovery of trioxane, in whichthe trioxane is transferred from a liquid mixture comprising trioxane,formaldehyde, alcohol, hemiformals formed from the formaldehyde and thealcohol, usual secondary components arising in the preparation oftrioxane, and, in addition or as an alternative to the alcohol and thehemiformals, water and reaction products formed from formaldehyde andwater, into the gas phase by volatilization or evaporation andsubsequently converted into a liquid state by condensation and isolatedas condensate or converted into a solid state by desublimation andisolated as desublimate, and wherein the condensate or the desublimatehas a trioxane content which is at least 2.5 times as high as thetrioxane content in the liquid mixture, and wherein the process iscarried out using a carrier gas.
 2. The process as claimed in claim 1,wherein the liquid mixture essentially comprises trioxane, formaldehyde,alcohol, hemiformals formed from the formaldehyde and the alcohol, usualsecondary components arising in the preparation of trioxane, and from 0to 5% by weight, preferably from 0 to 3% by weight, of water.
 3. Theprocess as claimed in claim 1, wherein the condensate or the desublimatehas a trioxane content of at least 80% by weight.
 4. The process asclaimed in claim 1, wherein the vaporization or evaporation is carriedout at a temperature in the range from 20 to 200° C., the vaporizationor evaporation optionally being carried out at reduced pressure,superatmospheric pressure or atmospheric pressure.
 5. The process asclaimed in claim 4, wherein the vaporization or evaporation is carriedout at a temperature in the range from 50 to 100° C.
 6. The process asclaimed in claim 1, wherein a condensation is carried out at atemperature in the range from 30 to 113° C., preferably at a temperaturein the range from 30 to 75° C., or a desublimation is carried out at atemperature in the range from −20 to 70° C., preferably at a temperaturein the range from 20 to 40° C.
 7. The process as claimed in claim 1,wherein the vaporization or evaporation and the condensation ordesublimation is optionally each carried out in a one-step or multistepprocess.
 8. The process as claimed in one of claims 1, 2 or 3-6 whereinthe vaporization or evaporation is carried out in at least one apparatusselected from evaporators, columns, stirred vessels, bubble-tray columnsand flash apparatuses, and the condensation or desublimation is carriedout in at least one apparatus selected from heat exchangers, fluid-bedapparatuses, fluidized-bed apparatuses, bubble-tray columns and stirredvessels.
 9. The process of claim 1 wherein the water content is 0 to 5%by weight.